When diving underwater, a diver needs a device which measures his depth underwater. A device which provides the diver an indication of depth can be of several types. A mechanical gauge has no electronic components, but is extremely inaccurate. In recent decades, electronic depth measurement devices have been used for improved accuracy. In addition to providing the diver with an indication of his depth underwater, information about the diver's depth can be used to calculate the necessary ascent schedule to avoid decompression sickness. As such, a device which can measure depth underwater has a multitude of purposes for the diver.
As the pressure underwater increases dramatically and linearly with depth, the traditional approach to measure depth is to measure the ambient pressure in the vicinity of the diver. This pressure can be easily and quickly converted into a depth by a microprocessor and rendered on a display to a diver.
Measuring pressure underwater is accomplished by a pressure transducer. Numerous different kinds of pressure transducers exist, such as piezoresistive silicon transducers, for example.
Piezoresistive silicon transducers consist of a small silicon die etched with pressure sensitive resistors. Extremely thin microwires are bonded to this die, and these microwires are then bonded to a larger package. When the pressure increases, the resistance of the resistors on the silicon die changes, and with proper signal processing, the change of resistance can be converted into a pressure.
The silicon die and the microwires bonded to the silicon directly experience the pressure to be measured, however, these components are not waterproof. Therefore, the die and microwires are protected from water by some medium that allows pressure to be transmitted through the medium. Known methods for waterproofing the components involves covering the die and microwires with an oil and sealing the oil with a stainless steel membrane, or, alternatively, potting the die and microwires with a very soft silicone gel.
When the die and microwires are immersed in oil and sealed with a steel membrane which transmits the pressure, the result is a product which has high accuracy and high stability. Two drawbacks of such a product are extremely high cost, and low mechanical shock survivability. An extreme mechanical shock can cause the microwires to break or debond from the die, causing the transducer to fail irreparably. The cost of the oil filled stainless steel membrane silicon die transducer is typically at least $60 USD even in high quantities, making it impractical for all except the most expensive underwater products.
Another known method of protecting the silicon die and microwires from water is to pot them with an extremely soft silicone gel. This has the advantage of greatly reducing the cost compared with the oil filled solution. A gel filled silicon piezoresistive sensor array may sell for $6-$15 USD in large quantities. Companies which are currently producing pressure transducers specifically for diving are all producing gel filled silicon piezoresistive pressure transducers. Unfortunately, gel filled piezoresistive silicon pressure transducers are extremely unreliable for numerous reasons.
Firstly, the gel used to pot the sensor, although very soft, is far more rigid than oil. The gel expands and contracts as it heats and cools during normal operation as the diver immerses himself in cold water, and then exposes himself to sunlight. As the gel expands and contracts, it pulls the microwires along with it, which can cause them to break. Further, strong mechanical shocks to the sensor create shockwaves which travel through the gel, which can create displacements within the gel, which can again break the microwires bonded to the die. In addition, silicone gels have inherently poor resistance to water and seawater. Silicone has a tendency to absorb moisture, and although numerous methods have been devised to reduce that tendency, the tendency cannot be completely eliminated. Silicone also has trouble bonding or sticking to various types of surfaces, and the curing of two part addition cure silicones can be inhibited by impurities on the surfaces to be bonded. Even when bonding perfectly clean surfaces, complicated ‘primer’ chemicals are needed to help the bonding process. Despite that, poor bonding can still occur. If the gel is not properly bonded to the die and microwires, then water can seep in between the gel and the die, causing a flood failure.
Given the numerous failure mechanisms of gel filled silicon sensors, and given that these sensors are by far the predominant ones used in the industry, then one would expect the industry to be filled with reports of pressure transducer failures in depth measuring underwater products. That is in fact exactly the case. Consulting any expert in the field or even performing a simple internet search will reveal that the predominant failure mechanism of underwater depth measuring products is failure of the pressure transducer. Given the numerous failure mechanisms of gel filled silicon sensors, and the extreme challenges to manufacture them around those failure mechanisms, one would expect variability from one batch to the next of such sensors. That is in fact the case, where numerous times in the industry, entire production runs of depth measuring instruments have been recalled or replaced due to faulty, defective, or poorly manufactured gel filled silicon sensors.
The failure rate of the standard sensors used in the industry is so high that most manufacturers of depth measuring devices have devised methods of trying to protect, reinforce or otherwise improve the performance of the gel filled silicon sensors. Alternatively, some products are designed such that a failed pressure transducer can be easily replaced by a technician. In that case, the entire product may be designed around ease of replacement of the pressure transducer, due to the high anticipated failure rate of the transducer.
Improvements in underwater depth measurement devices are desirable.